With the development of recent display techniques, there have been proposed and developed display apparatuses having a function other than a function of displaying a two-dimensional (plane) images. For example, a three-dimensional display which makes it possible to view not a two-dimensional image but a three-dimensional image can be said to be a typical example of such display apparatuses. As one of display apparatuses proposed as the three-dimensional display, there is a time-division three-dimensional display described in Patent Literatures 1 to 3. The principle of the time-division three-dimensional display will be described with reference to FIG. 1.
Referring to FIG. 1, the time-division three-dimensional display displays a left-eye image and a right-eye image constituting a three-dimensional image, switching the images at a high speed. An optical shutter such as liquid crystal glasses is constituted by a right-eye shutter and a left-eye shutter. In synchronization with the right-eye image being displayed, the right-eye shutter gets into a light transmission state and the left-eye shutter gets into a light blocking state. In synchronization with the left-eye image being displayed, the left-eye shutter gets into a light transmission state and the right-eye shutter gets into a light blocking state. When the three-dimensional display is viewed through the optical shutter, different images temporally enter the right and left eyes to realize three-dimensional display.
Another example of the display apparatuses having a function other than the function of displaying a two-dimensional image will be described. Content to be displayed on a display apparatus includes such content that should not be viewed by others, such as secret information and private data. These days when ubiquitous accompanying the development of information equipment is developing, it is an important to prevent display contents from being viewed by others, even among the public that includes an unspecified large number of people. As a related art to solve this problem, there is described an image display apparatus capable of preventing a display image from being stealthily viewed by others in Patent Literature 4. FIG. 2 shows the configuration of the image display apparatus.
Referring to FIG. 2, the image display apparatus described in Patent Literature 4 includes image information accumulation memory 202, synthesis circuit 205, brightness/saturation conversion circuit 206, image display device 208, glasses-shutter timing generation circuit 209, and glasses 211 such as liquid crystal glasses.
Image information accumulation memory 202 stores inputted image signal 201 in frames on the basis of frame signal 203. The image signal stored in image information accumulation memory 202 is read twice at a rate twice as fast as the frame cycle. The image signal read first is provided for synthesis circuit 205 as first image signal 204 compressed to one half. The image signal that is read the second time is provided for synthesis circuit 205 as second image signal 207 after having undergone saturation and brightness conversion processing by brightness/saturation conversion circuit 206. Output of synthesis circuit 205 is provided for image display device 208 as a display signal. Accordingly, an image based on first image signal 204 and an image based on second image signal 207 are alternately displayed on image display device 208.
Glasses-shutter timing generation circuit 209 generates glasses-shutter driving signal 210 for driving the shutter of glasses 211 on the basis of frame signal 203. Glasses-shutter driving signal 210 is a timing signal to cause the shutter of glasses 211 to be turned on (light blocking state) during a period while an image based on second image signal 207 is displayed. By the shutter of glasses 211 being driven by this glasses-shutter driving signal 210, only the image based on first image signal 204 (secret image) are presented to a person who wears glasses 211 can view.
A person who does not wear glasses 211 sees a gray image in which the image based on first image signal 204 and the image based on second image signal 207 are merged, due to the temporal integration effect of eyesight (persistence of vision). This gray image is an image quite different from the image based on first image signal 204 (secret image). Thus, the person who does not wear glasses 211 cannot identify the image based on first image signal 204 (secret image).
In the example shown in FIG. 2, the secret image is shown to a person who wears liquid crystal glasses and not shown to a person who does not wear the liquid crystal glasses. Another example of such images that are related to one another includes, a secret image which can be viewed only by a person who wears liquid crystal glasses, a reverse image to negate the secret image and make the secret image appear to be an image quite unrelated to the secret image, and a public image to be viewed by a person who does not wear the liquid crystal glasses. In this example, in order to prevent both of the persons who wears and who does not wear the liquid crystal glasses from perceiving flickers, it is required that the cycle of displaying the set of three images of the secret image, the reverse image and the public image should be 60 Hz or more, that is, the frame frequency for displaying each unit image (subframe) of the secret image, the reverse image and the public image should be 180 Hz or more, in consideration of the time integration effect in human eyesight. If an image is displayed with a frame frequency lower than 180 Hz, the image is easily perceived as flicker by human eye, and thus, there is a problem that the image quality deteriorates. Furthermore, if an image is displayed with a frame frequency lower than 180 Hz, each secret image, the reverse image and public image appear to be independently displayed, and consequently, even the person who does not wear the liquid crystal glasses can view the secret image. Thus, there is a problem that the secrecy of the secret image deteriorates.
In order to prevent flicker, it is required, in the example shown in FIG. 2, to combine a secret image and a gray image as a set and set the frequency for displaying the set to 60 Hz or more similarly and, in the example of the three-dimensional shown in FIG. 1, to combine a right-eye image and a left-eye image as a set and set the frequency for displaying the set to 60 Hz or more similarly. In these cases, it is necessary to display each unit image (subframe) with a frame frequency of 60×2=120 Hz.
Thus, in order to realize a display apparatus utilizing the time integration effect of eyesight, it is required to display a unit image (subframe) constituting a frame, such as the secret image and the right-eye image, with a high-speed frame frequency of 120 Hz or more. To achieve this, it is desirable to, when transmitting these images to a display apparatus from an image transmission source (a PC (Personal Computer) or the like), transmit them with the same frame frequency as the high-speed frame frequency, such as 120 Hz and 180 Hz.
However, in image transmission standards which are currently widely prevalent, such as the DVI, the limit of the frame frequency capable of transmitting an image is practically 60 Hz. Therefore, it is not possible to transmit an image with a frame frequency more than 60 Hz. As measures for transmitting an image with a frame frequency more than 60 Hz, measures of devising a new image transmission system compatible with the high-speed frame frequency as described above and measures of parallelizing existing image transmission paths are conceivable.
However, the former measures have a problem that the versatility is low because of the specialization in high-speed transmission, in addition to the problem of the enormous cost of new development of compatible chips (for a transceiver and a receiver) and cables. The latter measures have a problem in which the wiring of cables is complicated. Therefore, neither measure can be said to be a realistic solution. Thus, it is desirable to transmit an image using only one image transmission path of an existing image transmission standard.
As an example of related art to realize this, the technique described in Patent Literature 5 can be given (FIG. 3). This technique relates to a technique of transmitting an image to a display apparatus which displays a three-dimensional image by a system different from the time-division system as shown in FIG. 1. In the technique described in Patent Literature 5, two-dimensional image 301 and depth image 302 are multiplexed to one large image by multiplexing means 312, and the multiplexed image is transmitted to a display apparatus with the use of an existing one two-dimensional image transmission path, as shown in FIG. 3. On the display apparatus side, separation means 313 separates the multiplexed image into two-dimensional image 301 and depth image 302.